Resistor array with position dependent heat dissipation

ABSTRACT

A thermally actuated fluidic optical switching circuit that includes a heater substructure having heater resistors and thermally conductive regions associated with the heater resistors and configured to tailor the thermal characteristics of the heater substructure.

BACKGROUND OF THE INVENTION

[0001] The disclosed relates generally to optical switching circuits,and more particularly to optical switching circuits that employ heaterresistors to control the states of optical switching elements.

[0002] Optical fibers are replacing conductive wires in telephone anddata communications, since optical fibers provide extremely highbandwidth, are immune to radio frequency noise, and generate virtuallyno electromagnetic interference. As the cost of optical fibersdecreases, use of optical fibers is expanding to applications thatrequire switching to dynamically reconfigure the interconnection ofoptical signal paths.

[0003] A known approach to optical switching involves thermallycontrolling the presence or absence of liquid in a gap at which aplurality of optical waveguide segments intersect. This approach can beimplemented for example in an optical switching circuit that includes awaveguide substrate having a plurality of thermally actuated fluidicoptical switches, and a heater substrate disposed adjacent the waveguidesubstrate. The heater substrate includes an array of heater resistorsthat selectively thermally actuate the optical switches, for example byforming drive bubbles to move fluid to move into and out of gaps in thewaveguide substrate that transmit or reflect light as a function of thepresence or absence of fluid.

[0004] A consideration with the foregoing fluidic optical switchingcircuit is the non-uniform thermal characteristics of heater resistorsin the heater substrate. For example, resistors closer to the middle ofthe heater substrate have less heat capacity than resistors closer tothe edges of the heater substrate. The non-uniform thermalcharacteristics may degrade performance, and may also lead toreliability issues for the resistors located near the center of theheater substrate.

[0005] There is accordingly a need for an optical switching circuitheater resistor array having localized heat dissipation characteristicsthat are individually adjustable.

SUMMARY OF THE INVENTION

[0006] The disclosed invention is directed to a heater resistor arraythat includes a thin film integrated circuit sub-structure, a pluralityof heater resistors defined in the thin film sub-structure, and aplurality of thermally conductive regions defined in the thin filmsub-structure and dielectrically separated from said heater resistors,each of the thermally conductive regions being located proximately to anassociated one of the heater resistors for dissipating heat from theassociated heater resistor, wherein each of the thermally conductiveregions has an area that is selected to adjust or tailor the heatdissipation capacity of the associated heater resistor. In a particularembodiment of the invention, the heater resistors are planar heaterresistors while the thermally conductive regions are planar metalregions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The advantages and features of the disclosed invention willreadily be appreciated by persons skilled in the art from the followingdetailed description when read in conjunction with the drawing wherein:

[0008]FIG. 1 is a schematic elevational view illustrating a thermallyactuated fluid optical switching circuit in which the invention can beemployed.

[0009]FIG. 2 is a schematic top plan view illustrating a heater resistorarray of the optical switching circuit of FIG. 1.

[0010]FIG. 3 is an unscaled schematic cross sectional view of an exampleof a heater substructure of the circuit of FIG. 1 taken laterallythrough a representative heater resistor region.

[0011]FIG. 4 is an unscaled schematic cross sectional view of a furtherexample of a heater substructure of the circuit of FIG. 1 takenlaterally through a representative heater resistor region.

[0012]FIG. 5 is a top plan view illustrating a via structure of theheater substructure of FIG. 4.

[0013]FIG. 6 is a top plan view illustrating another via structure ofthe heater substructure of FIG. 4.

[0014]FIG. 7 is an unscaled schematic cross sectional view of anotherexample of a heater substructure of the circuit of FIG. 1 takenlaterally through a representative heater resistor region.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0015] In the following detailed description and in the several figuresof the drawing, like elements are identified with like referencenumerals.

[0016] Referring now to FIG. 1, set forth therein is a schematicelevational block diagram of a thermally actuated fluidic opticalswitching circuit in which the invention can be employed and whichgenerally includes a waveguide substrate 13 having a plurality ofthermally actuated fluidic optical switches 21, and an integratedcircuit or thin film heater substructure or die 11 disposed adjacent oneside of the waveguide substrate. The thin film heater substructure ordie 11 includes heater resistors 56 defined therein for thermallyactuating the fluidic optical switches in the waveguide substrate 13,and generally comprises a substrate such as silicon and thin film layersformed thereon. The thermally actuated fluidic optical switching circuitof FIG. 1 can also include a further thin film heater substructure 15(shown in broken lines) on the other side the waveguide substrate 11.Examples of thermally actuated fluidic optical switching circuits inwhich the invention can be incorporated are disclosed in U.S. Pat. No.5,699,462, Fouqet et al., incorporated herein by reference.

[0017] Referring now to FIG. 2, schematically set forth therein is a topplan view of the thin film heater substructure 11 that illustrates aheater resistor structure array in accordance with the invention. Thethin film heater substructure 11 includes a plurality of resistors 56,and a plurality of thermally conductive regions 58 respectivelyassociated with and underlying the heater resistors 56 for dissipatingheat from the associated heater resistors 56. By way of illustrativeexample, as described further herein, the heater resistors 56 comprisethin film resistors, while the thermally conductive regions comprisemetal regions or slabs. In FIG. 2 the thermally conductive regionsassociated with heater resistors close to the edges of the thin filmheater substructure are not shown so as to indicate that such thermallyconductive regions do not extend laterally beyond their associatedheater resistors.

[0018] The thermally conductive regions thermally couple the heaterresistors to the substrate of the thin film heater sub-structure 11, andin accordance with the invention, each of the thermally conductive heatdissipating regions 58 is individually configured to tailor or adjustthe heat dissipating capacity of the particular heater resistorstructure formed by a particular heater resistor and an associatedthermally conductive region, for example to configure the thermalprofile of the thin film heater substructure 11. In particular, the heatdissipating capacity of a thermally conductive region increases as thearea thereof increases, and the heat capacity of a heater resistorstructure is adjusted by adjusting the area of the thermally conductiveregion of the particular heater resistor structure.

[0019] By way of a specific example wherein mounting and sealingstructures are attached to edges of the thin film heater substructure,thermally conductive regions 58 in the middle portion of the thin filmheater substructure 11 can be configured to have greater areas thanthermally conductive regions 58 near the edges of the thin film heatersubstructure 11, for example, so as to provide for a more uniformthermal characteristic across the thin film heater substructure 11. Thisoccurs since the heater resistors near the edges of a thin film heatersubstructure 11 having mounting and sealing structures attached to itsedges would have greater heat dissipation capacities due to closerproximity to the mounting and sealing structures, while heater resistorsin the middle of the thin film heater substructure would have less heatcapacity due to the relatively poor thermal conductivity of thedielectric layers of the thin film heater substructure.

[0020] By way of another example wherein a mounting structure isattached to the middle portion of the thin film heater substructure 11,the thermally conductive regions 58 near the edges of the thin filmheater substructure 11 can be configured to have greater areas thanthermally conductive regions 58 in the middle portion of the thin filmheater substructure 11, for example, so as to provide for a more uniformthermal characteristic across the thin film substructure 11.

[0021] In general, it should be appreciated that the thermal dissipationcharacteristics, pattern, or profile of the thin film heatersubstructure 11 is tuned by varying or selecting the areas of thethermally conductive regions as a function of location in the thin filmsub-structure.

[0022] The thin film heater substructure 11 can be made pursuant tostandard thin film integrated circuit processing including chemicalvapor deposition, photoresist deposition, masking, developing, andetching, for example as disclosed in commonly assigned U.S. Pat. No.4,719,477 and U.S. Pat. No. 5,317,346, incorporated herein by reference.

[0023] Referring now to FIG. 3, set forth therein is an unscaledschematic cross sectional view of a particular implementation of theheater substructure 11 taken through a representative heater resistor56. The thin film heater substructure 11 more particularly includes asilicon substrate 51, a thermally grown silicon dioxide layer 53disposed over the silicon substrate 51, and a patterned metallizationlayer including metal subareas or slabs 58 disposed on the thermal oxidelayer 53. A deposited silicon dioxide layer 54 is disposed over thefirst metallization layer that includes the metal sub-areas 58, while aresistive layer 55 comprising for example tantalum aluminum is formed onthe deposited oxide layer 54. A patterned metallization layer 57comprising aluminum doped with a small percentage of copper and/orsilicon, for example, is disposed over the resistive layer 55.

[0024] The metallization layer 57 comprises metallization traces definedby appropriate masking and etching. The masking and etch of themetallization layer 57 also defines the resistor areas. In particular,the resistive layer 55 and the metallization layer 57 are generally inregistration with each other, except that portions of traces of themetallization layer 57 are removed in those areas where heater resistorsare formed. In this manner, the conductive path at an opening in a tracein the metallization layer 57 includes a portion of the resistive layer55 located at the opening or gap in the conductive trace. Stated anotherway, a resistor area is defined by providing first and second metallictraces that terminate at different locations on the perimeter of theresistor area. The first and second traces comprise the terminal orleads of the resistor which effectively include a portion of theresistive layer that is between the terminations of the first and secondtraces. Pursuant to this technique of forming resistors, the resistivelayer 55 and the metallization layer can be simultaneously etched toform patterned layers in registration with each other. Then, openingsare etched in the metallization layer 57 to define resistors. The heaterresistors 56 are thus particularly formed in the resistive layer 55pursuant to gaps in traces in the metallization layer 57.

[0025] The metal subareas 58 underlie and are proximate to associatedheater resistors 56, and comprise heat dissipating thermally conductiveregions. In accordance with the invention, the areas of the metalsubareas, which are generally planar, are individually configured toachieve a desired heat dissipation capacity for the laminar heaterresistor structure formed by the heater resistor and the associatedmetal subarea. This allows for tailoring of the thermal characteristicof the thin film heater substructure 11.

[0026] A composite passivation layer comprising a layer 59 of siliconnitride (Si₃N₄) and a layer 60 of silicon carbide (SiC) is disposed overthe metallization layer 57, the exposed portions of the resistive layer55, and exposed portions of the oxide layer 53. Optionally, a tantalumpassivation layer that includes tantalum subareas 61 can be disposed onthe composite passivation layer 59, 60 over the heater resistors 56, forexample to provide for mechanical passivation that absorbs cavitationpressure of collapsing drive bubbles produced in the fluid in thewaveguide substrate 13 pursuant to selective energizing of the heaterresistors.

[0027] Referring now to FIG. 4, schematically set forth therein is aschematic cross sectional view of a further implementation of the heatersubstructure 11 taken through a representative heater resistor 56. Theheater substructure 11 of FIG. 4 is similar to the heater substructure11 of FIG. 3 and includes metal slabs 58 that are electrically contactedto the silicon substrate 51 by vias 158. The vias 158 can comprisecylindrical vias as illustrated in FIG. 5, or line vias as illustratedin FIG. 6. The vias 158 are formed for example by etching via openingsin thermal oxide layer 53 prior to deposition of the first metallizationlayer in which the slabs 58 are formed. In accordance with theinvention, the area of each of the metal slabs 58 is individuallyconfigured to define a desired thermal dissipation capacity for theparticular heater resistor structure.

[0028] Referring now to FIG. 7, schematically set forth therein is aschematic cross sectional view of another implementation of the heatersubstructure 11 taken through a representative heater resistor 56. Theheater substructure 11 of FIG. 7 is similar to the heater substructure11 of FIG. 3 and includes metal slabs 58 that are disposed on thesilicon substrate 51 and thereby electrically contacted to the siliconsubstrate 51. The metal slabs 58 are basically large contacts, and areformed for example by etching suitable openings in thermal oxide layer53 prior to deposition of the first metallization layer in which themetal slabs 58 are formed. In accordance with the invention, the area ofeach of the metal slabs 58 is individually selected to define a desiredthermal dissipation capacity for the particular heater resistorstructure.

[0029] It should be appreciated that the thin film heater substructure11 can include active devices, in which case additional layers would beformed between the formation of the thermal oxide layer 53 and formationof the metallization layer that includes the metal slabs 58. Forexample, poly silicon would be deposited and patterned on the thermaloxide layer, and a doped oxide layer would be deposited, densified andreflowed. The first metallization layer would then be deposited andpatterned.

[0030] The foregoing has thus been a disclosure of a heatersub-structure that is useful for optical switching circuits and whichadvantageously includes heater resistors having individually tailoredheat dissipation characteristics.

[0031] Although the foregoing has been a description and illustration ofspecific embodiments of the invention, various modifications and changesthereto can be made by persons skilled in the art without departing fromthe scope and spirit of the invention as defined by the followingclaims.

What is claimed is:
 1. An optical switching circuit comprising: a thinfilm substructure; a plurality of heater resistors formed in said thinfilm substructure; a plurality of thermally conductive regionsrespectively associated with said heater resistors and dielectricallyseparated from said heater resistors, each of said thermally conductiveregions being located proximately to an associated one of said heaterresistors for dissipating heat from said associated heater resistor,wherein said thermally conductive regions have respective areas that areselected so as to tailor respective heat dissipation capacities ofassociated ones of said heater resistors; and a waveguide substrateadjacent to said thin film substructure having a plurality of fluidicoptical switching elements that are actuated by thermal energy from saidheater resistors.
 2. The optical switching circuit of claim 1 whereinsaid heater resistors comprise planar heater resistors, and wherein saidthermally conductive regions comprise planar thermally conductiveregions.
 3. The optical switching circuit of claim 2 wherein said planarthermally conductive regions comprise planar metal regions.
 4. Theoptical switching circuit of claim 3 wherein said planar heaterresistors are separated from said planar metal regions by silicondioxide.
 5. The optical switching circuit of claim 4 wherein said thinfilm substructure includes a silicon substrate.
 6. The optical switchingcircuit of claim 1 wherein said thin film substructure includes asilicon substrate, and wherein said thermally conductive regionscomprise metal regions.
 7. The optical switching circuit of claim 6wherein said metal regions are contacted to said silicon substrate. 8.The optical switching circuit of claim 7 further including metal viasfor contacting said metal regions to said silicon substrate.
 9. Theoptical switching circuit of claim 7 wherein said metal regions comprisemetal contact slabs.
 10. The optical switching circuit of claim 1wherein said areas of said thermally conducting regions vary dependingon locations of respective thermally conductive regions in said thinfilm substructure.
 11. The optical switching circuit of claim 10 hereinthermally conductive regions in a middle portion of said thin filmsubstructure have respective areas that are greater than areas ofthermally conductive regions near edges of said thin film substructure.12. The optical switching circuit of claim 10 herein thermallyconductive regions in a middle portion of said thin film substructurehave respective areas that are smaller than areas of thermallyconductive regions near edges of said thin film substructure.
 13. Aheater resistor array comprising: a thin film substructure; a pluralityof thin film heater resistors formed in said thin film substructure; anda plurality of metal regions respectively associated with said heaterresistors and dielectrically separated from said heater resistors, eachof said metal regions being located proximately to an associated one ofsaid heater resistors for dissipating heat from said associated heaterresistor, wherein said metal regions have respective areas that areselected so as to tailor a heat dissipation characteristic of said thinfilm substructure.
 14. The heater resistor array of claim 13 whereinsaid said areas of said metal regions vary depending on location in saidthin film substructure.
 15. The heater resistor array of claim 13wherein said thin film substructure includes a silicon substrate. 16.The heater resistor array of claim 15 wherein said metal regions arecontacted to said silicon substrate.
 17. The heater resistor array ofclaim 16 further including metal vias for contacting said metal regionsto said silicon substrate.
 18. The heater resistor array of claim 16wherein said metal regions comprise metal contact slabs disposed on saidsilicon substrate.